The present invention relates to autofocus devices, and more particularly, to an autofocus device including a software improved for focus tracking control of high accuracy in image sensing apparatus such as a video camera.
In the autofocus control of image sensing apparatus such as a video camera, focus tracking control is carried out according to each zoom position on the basis of a zoom tracking curve indicating the relationship between the zoom position of a group of zoom lens and the focus position of a group of focus lens. A table of the data of the zoom tracking curve is stored in the memory of the autofocus device.
The zoom tracking curve differs according to the distance to a subject of interest. If tracking control of focus in high accuracy is to be carried out with respect to every subject distance, a table including all zoom tracking curves corresponding to every subject distance must be provided. Therefore, a great amount of storage capacity for data will be required.
Accordingly, only a table storing several typical zoom tracking curves, i.e., zoom tracking curves for the cases where the distance up to a subject corresponds to an infinity point and to a nearby point is provided, and zoom tracking curves corresponding to other subject distance are obtained by interpolation processing that geometrically divides between such prestored zoom tracking curves.
The calculated value of a zoom tracking curve obtained from the above-described interpolation processing by geometric division of a few number of zoom tracking curves substantially matches the design value obtained definitely for each subject distance in the design stage as far as the axis of lens and the proximity thereof is concerned.
However, in an area distant in the radial direction from the axis of lens, the error between the calculated value by interpolation processing and the design value obtained by actual designing will become greater due to spherical aberration of lens to result in a noticeable out-of focus state.
Such an error is particularly significant with respect to a subject distance that is around the middle of respective subject distance corresponding to the plurality of, for example 2, prestored zoom tracking curves used as the basis for interpolation processing. The trend of a noticeable defocused state is noted.
For the purpose of reducing such an error between a calculated value and a design value to improve the accuracy of focus tracking, a table with a greater number of zoom tracking curves must be provided. However, this induces the problem of increase in the data storage capacity and complication in data processing.
Therefore, an object of the present invention is to provide an autofocus device that has the focus tracking accuracy improved significantly without having to greatly increase the data storage capacity.
According to the invention defined in claim 1, an autofocus device of image sensing apparatus includes memory means for storing data of a plurality of zoom tracking curves corresponding respectively to a plurality of different subject distance and precalculated correction data, calculation means carrying out a predetermined interpolation process using said plurality of zoom tracking curves and said correction data for calculating a zoom tracking curve with respect to a subject distance differing from said plurality of subject distance, and tracking means for tracking focus according to a calculated zoom tracking curve.
According to the invention defined in claim 2, an autofocus device of image sensing apparatus includes memory means for storing data of a plurality of zoom tracking curves corresponding respectively to a plurality of different subject distance and correction data calculated according to a zoom tracking curve obtained by a predetermined interpolation process with respect to a predetermined subject distance that is intermediate said plurality of subject distance, calculation means carrying out a predetermined interpolation process using said data of a plurality of zoom tracking curves and said correction data for calculating a zoom tracking curve with respect to a subject distance differing from said plurality of subject distance, and tracking means for tracking focus according to said calculated zoom tracking curve.
According to the invention defined in claim 3, said calculation means in the autofocus device of claim 2 includes means for calculating a focus position which is said correction data subtracted from a current focused position, means for geometric-dividing said plurality of zoom tracking curves on the basis of said calculated focus position, and means for adding said correction data to said zoom tracking curve obtained by geometric division for obtaining a zoom tracking curve with respect to said different subject distance.
According to the invention defined in claim 4, said correction data is calculated according to the error between said zoom tracking curve obtained with respect to said intermediate predetermined subject distance and the design value in the autofocus device of claims 1-3.
According to the invention defined in claim 5, a method of correcting a zoom tracking curve in an autofocus device includes the steps of: preparing memory means in which are stored data of a plurality of zoom tracking curves respectively corresponding to a plurality of different subject distance and correction data calculated according to a zoom tracking curve obtained by a predetermined interpolation process with respect to a predetermined subject distance intermediate said plurality of subject distance, calculating a focus position which is said correction data subtracted from a current focused position, geometric-dividing said plurality of zoom tracking curves on the basis of said calculated focus position, and adding said correction data to said curve obtained by geometric division for obtaining a zoom tracking curve for a subject distance differing from said plurality of subject distance.
According to the invention defined in claim 6, said correction data is calculated according to the error between the zoom tracking curve obtained for said intermediate predetermined subject distance and a design value in the correction method of claim 5.